Many applications arise in which it is necessary to measure the level of liquid in a container. For example, it is necessary to monitor oil, gasoline, and water levels in various types of storage situations such as on cars, trucks, and other vehicles. It is also often necessary to monitor the liquid level of various other chemicals such as corrosive materials in their storage tanks. Although mechanical level monitors are often used in these situations, it would be desirable to have an alternative means which could prove less expensive, more convenient to operate and monitor, more reliable, and one which could function in a hostile environment.
Several attempts have been made to employ the modulation of light in an optical fiber in order to sense liquid levels. In one instance a prism was attached to the end of a fiber such that, if liquid contacted the prism, light was not reflected back into the fiber. This method has the problem that the prism is a rather expensive optical element, and it must be aligned with and cemented to the fiber. Also, the light-transmitting capability of such a system in the non-immersed condition is low.